The present invention relates to a superconducting current detecting circuit suitable for a high-sensitive fluxmeter using a Josephson device.
A fluxmeter using a Josephson device which is called a superconducting quantum interference device (hereinafter referred to as a SQUID fluxmeter), is known in the technical field of a fluxmeter. As the SQUID fluxmeter, a DC-SQUID fluxmeter and an AC-SQUID fluxmeter are typically known, the former being arranged such that magnetic flux interlinked with a superconducting loop constituted by two Josephson junctions and an inductor is measured by changes in maximum DC superconducting current flowing across the superconducting loop, the latter being arranged such that magnetic flux interlinked with a superconducting loop constituted by one Josephson junction and an inductor is measured by changes in maximum AC superconducting current flowing in the superconducting loop. In any one of the conventional SQUID fluxmeters, a main component thereof, is constituted by a passive element for detecting magnetic flux interlinked with the superconducting loop. The SQUID employing a Josephson junction is dipped in liquid helium and an output signal from the SQUID is transferred through a distribution cable to measuring and data processing circuits maintained at a room temperature. The SQUID acts to measure exceedingly weak magnetic flux, and the output signal of the Josephson device per se is weak, so that the signal transferred from the SQUID to the measuring circuit maintained at a room temperature is extremely weak to be, for example, about 1 .mu.V. Therefore, sensitivity of the conventional SQUID fluxmeter has been limited by thermal agitation noises at a room temperature.